The present invention relates to a method for preparing non-photosensitive fatty acid silver salt grains used for composing a photothermographic material, and an apparatus for implementing the method.
A strong need for reducing the volume of waste process solution has arisen in recent medical field from viewpoints of environmental preservation and space saving. Thus a technology related to a photosensitive photothermographic material for medical diagnosis and photographic purposes has been desired, the material being such that affording efficient light exposure with a laser image setter or laser imager, and providing a black image with high resolution and sharpness. Such photosensitive photothermographic material can provide the user with a more simple and environment-conscious image producing system using no solution-base process chemical.
The image producing method based on heat development is disclosed, for example, in U.S. Pat. Nos. 3,152,904 and 3,457,075 and xe2x80x9cThermally Processed Silver Systemsxe2x80x9d written by D. Morgan and B. Shely, Imaging Processes and Materials, Neblette""s 8th ed., edited by Sturge, V. Walworth and A. Shepp, p.2, (1989).
Such photosensitive material contains an reducible non-photosensitive silver source (e.g. , organic acid silver salt), a catalytic amount of photocatalyst (e.g., silver halide) and a reducing agent for silver, all of which being generally dispersed in an organic binder matrix. While the photosensitive material is stable at the room temperature, it will produce silver through a redox reaction between the reducible silver source (which serves as an oxidizing agent) and the reducing agent when heated to a high temperature (80xc2x0 C. or above, for example) after light exposure. The redox reaction is promoted by a catalytic action of the latent image produced by the light exposure. That is, the silver generated by the reaction of the reducible silver within the exposed area provides a black spot, which makes a contrast with the non-exposed area and is recognizable as an image.
The silver source employed by such system generally refers to a silver salt of a fatty acid, and a variety of methods for producing thereof have been known. Examples of the methods include such that preparing an organic acid silver salt in a concomitant solution of water and water-insoluble solvent as disclosed for example in JP-A-49-93310 (the code xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d), JP-A-49-94619 and JP-A-53-68702; such that preparing an organic acid silver salt in an aqueous solution as disclosed in JP-A-53-31611, JP-A-54-4117 and JP-A-54-46709; and such that preparing an organic acid silver salt in an organic solvent as disclosed in JP-A-57-186745, JP-A-47-9432 and U.S. Pat. No. 3,700,458. In principle, the organic acid silver salt is obtained by dissolving a fatty acid into water under heating to a temperature of the melting point thereof or above, adding sodium hydroxide or an alkali metal salt under vigorous stirring, and further adding silver nitrate to convert an alkali soap into a silver soap.
The alkali soap forms micell in the aqueous solution, which appears as a milky liquid. The conversion reaction from such micellar state to silver salt, however, often suffers from a problem in production stability. Thus as a measure for obtaining a homogeneous solution of alkali soap, use of a mixed solution of water and alcohol as a solvent is disclosed in JP-A-55-40607.
Now the alkali soap shows alkalinity as its name suggests, and is prepared under a high pH environment. Adding silver nitrate to an alkali solution, however, not only produces silver oxide as a by-product but also results in an undesirable production of silver nucleus by an action of a trace amount of contaminant which inevitably generates during the production and exhibits a high reducing activity under such high-pH environment. Such by-product is quite disadvantageous in that degrading property of the photothermographic material and in particular in that causing undesirable fog. From this viewpoint, a method for obtaining a homogeneous solution to suppress the generation of the by-product is disclosed in JP-A-55-40607, in which fog still remains unsolved.
In JP-A-9-127643, disclosed is a method for producing silver salt based on simultaneous measuring and addition of an alkali metal salt solution and silver nitrate solution, and is specified as simultaneous addition of aqueous sodium behenate solution and isopropyl alcohol. While the method is successful in at least lowering the high pH during the reaction to the medium range and thereby in suppressing the generation amount of silver oxide, fog still cannot totally be cleared due to a weak reducibility of isopropyl alcohol.
As described above, preparation of fatty acid silver salt needs particular accounts such that eliminating as possible reducible substances during the formation of fatty acid silver salt, controlling the grain size and controlling the grain form, where all of them cannot be satisfied at a time by the conventional method.
In the production of a photothermographic material using the fatty acid silver salt, a photosensitive layer thereof is often formed by coating a coating liquid containing an organic solvent such as toluene, methyl ethyl ketone or methanol. Using an organic solvent as the solvent, however, is not only disadvantageous in terms of safety in the production processes, adverse effects on human body, and high cost ascribable to the solvent recovery or the like, but is also inappropriate in terms of providing an environment-conscious photothermographic material.
Thus a method for forming the photosensitive layer using a water-base coating liquid is proposed. For example, JP-A-49-52626 and JP-A-53-116144 disclose cases using gelatin as a binder. In JP-A-50-151138, a case using polyvinyl alcohol as a binder is described.
A case with a combined use of gelatin and polyvinyl alcohol is found in JP-A-60-61747. As another exemplary case, the photosensitive layer using a water-soluble polyvinyl acetal as a binder is described in JP-A-58-28737.
As is clear from the above, using a water-soluble binder allows the photosensitive layer to be formed with a water-base coating liquid and is beneficial from environmental and economic viewpoints. The water-soluble polymer binder is, however, less compatible with the fatty acid silver salts, which will fail in obtaining a coated film with a surface quality agreeable to the practical use, will result in brownish to yellowish tone of the silver image after the development afar from intrinsically preferable black tone, and will result in increased fog. Thus only afforded was a photothermographic material whose property being significantly degraded and commercially unsuccessful.
In order to obtain practically agreeable quality of the coated surface using the water-base coating liquid containing a fatty acid silver salt, the fatty acid silver salt must be kept in a finely dispersed state in the water-base solution without causing agglomeration. Discovery of a method for finely dispersing the fatty acid silver salt is thus desired. One method generally accepted relates to such that producing a hydrophobic grain dispersion of a fatty acid silver salt, separating the grain therefrom by filtration to obtain a solid matter, and re-dispersing the solid matter after being mixed with a dispersing agent as described by D. Kloosterboer in Imaging Processes and Materials, Neblette""s 8th ed., edited by Sturge, V. Walworth and A. Shepp, p.279, (1989).
Fine dispersion operation of the fatty acid silver salt can be effected by mechanical dispersion in the presence of a dispersing agent using a known pulverizing means (e.g., high-speed mixer, homogenizer, high-speed impact mill, banbury mixer, homomixer, kneader, ball mill, vibration ball mill, epicyclic ball mill, attritor, sand mill, bead mill, colloid mill, jet mill, roller mill, trommel and high-speed stone mill). These methods, however, produce only a coating liquid including a lot of agglomerated grains and are thus causative of degraded surface quality, and, worse than all, tend to indiscriminately cleave the primary grains of the fatty acid silver salt which are originally crystallized as a water-insoluble salt, so that excessive silver nuclei are generated on the crystal cleavage plane of the grains and thereby to increase fog.
On the other hand, JP-B-7-119953 (the code xe2x80x9cJP-Bxe2x80x9d as used herein means an xe2x80x9cexamined Japanese Patent Publicationxe2x80x9d), JP-A-8-137044 and JP-A-8-238848 disclose methods such that finely dispersing the fatty acid silver salt by pressure treatment. The methods, however, relate to an organic solvent-base dispersion and stand on a concept different from solving the foregoing problem.
In JP-A-9-127643, disclosed is a method such that obtaining a dispersion of the fatty acid silver salt by simultaneous measuring and addition of an alkali metal salt solution and silver nitrate solution, and then directly desalting the dispersion by dialysis or ultra-filtration. This method is preferable at least in that the primary grain obtained in the crystallization process of the fatty acid silver salt can be incorporated as intact into the photosensitive layer without being crushed. The method, however, still suffers from problems in agglomeration of the grains under a condition of high salt concentration, and in thickening during concentration of the dispersion, which makes the method difficult to be accepted as a measure for obtaining a practical coating liquid.
Another problem resides in that vigorous stirring is required when the alkali metal salt solution and silver nitrate solution are mixed in order to obtain a fine and monodisperse grains of the fatty acid silver salt. In particular, since a solution of a fatty acid alkali metal salt dissolved at a high temperature will instantaneously deposit crystal due to abrupt cooling upon the addition, a slow dilution speed and moderate fluidization will undesirably result in large and coarse grains. Raising the stirring speed during the addition into a tank in which a gas/liquid interphase is formed, however, causes entrainment of the air. Since the fatty acid silver salt grains are strongly hydrophobic and will adhere on the surface of the entrained air bubbles, which not only prevents bubble rupture but also causes agglomeration of adjacent grains on the surface of the bubbles. The liquid such entraining the air appears like a whipped cream, and for the case of desalting the by-produced salt through ultra-filtration, this will significantly degrade the handling property, and the agglomerated grains will clog the filtration membrane.
Temperature of the reaction liquid after the reaction between the silver ion-containing solution and the solution of a fatty acid alkali metal salt is preferably kept around the room temperature, since too high temperature will result in growth of the grains by a physical ripening process. Whereas, the temperature needs be kept at 50xc2x0 C. or above to obtain a stable solution of an alkali metal salt of a long-chained fatty acid, so that it is necessary to ensure a rapid heat exchange so as to cancel the heat introduced with the added liquid. In this point, a measure for providing a jacket vessel to a tank or the like suffers from a problem in that a heat-exchangeable area reduces as volume of the reaction liquid increases.
As described above, a stable method for preparing a water-base coating liquid containing fatty acid silver salt grains capable of affording an excellent coated surface quality and optical properties such as low haze and low fog has not been discovered yet.
It is therefore an object of the present invention to provide a method for preparing fatty acid silver salt grains exhibiting an excellent dispersion stability and coating property when made into a dispersion, and can provide, when incorporated into a photothermographic material, an excellent fog preventive property during storage, and an excellent image stability and light transmissivity after the heat-development process.
The present inventors found after extensive investigations for achieving the above object that excellent non-photosensitive fatty acid silver salt grains can be obtained by mixing and reacting a silver ion-containing aqueous solution with a solution of a fatty acid alkali metal salt within a closed mixing means.
The xe2x80x9cclosed mixing meansxe2x80x9d as described herein refers to a means such that the inner space of which being filled with the liquids to be mixed, and having substantially no air phase, or in other words, having no gas/liquid interphase.
That is, the present invention is to provide a method for preparing non-photosensitive fatty acid silver salt grains having the step of reacting a silver ion-containing solution, the solvent of which being water, or a mixture of water and an organic solvent, with a solution of a fatty acid alkali metal salt, solvent of which being water, an organic solvent, or a mixture of water and an organic solvent, to obtain a fatty acid silver grain; in which the reaction is proceeded by mixing the silver ion-containing solution and the solution of the alkali metal salt of the fatty acid within a closed mixing means.
In the method for preparing non-photosensitive fatty acid silver salt grains of the present invention, it is preferable to further charge water, or a mixture of water and an organic solvent into the closed mixing means. It is particularly preferable that the water or the mixture contains a dispersing agent. It is also preferable to circularly feed back at least a part of a reacted mixture obtained after the reaction to the closed mixing means. It is still also preferable to cool the reacted mixture obtained after the reaction.
The present invention is to provide also an apparatus for preparing a non-photosensitive fatty acid silver salt grains having; a first feed means for feeding a silver ion-containing solution, the solvent of which being water or a mixture of water and an organic solvent, to a closed mixing means described later; a second feed means for feeding a solution of a fatty acid alkali metal salt, solvent of which being water, an organic solvent, or a mixture of water and an organic solvent, to the closed mixing means; a third feed means for feeding water, or a mixture of water and an organic solvent to the closed mixing means; and the closed mixing means for mixing matters fed from the first feed means, the second feed means and the third feed means, and discharging a liquid containing non-photosensitive fatty acid silver salt grains. It is preferable that the apparatus additionally has a cooling means for cooling the liquid containing non-photosensitive fatty acid silver salt grains discharged from the closed mixing means.
The present invention is to provide still also an apparatus for preparing non-photosensitive fatty acid silver salt grains having; a first feed means for feeding a silver ion-containing solution, the solvent of which being water or a mixture of water and an organic solvent, to a closed mixing means described later; a second feed means for feeding a solution of a fatty acid alkali metal salt, solvent of which being water, an organic solvent, or a mixture of water and an organic solvent, to the closed mixing means; the closed mixing means for mixing matters fed from the first feed means, the second feed means and a third feed means described next, and discharging a liquid containing non-photosensitive fatty acid silver salt grains; and the third feed means for feeding back at least a part of the liquid containing non-photosensitive fatty acid silver salt grains discharged from the closed mixing means to said closed mixing means. The apparatus preferably has a cooling means for cooling the liquid containing non-photosensitive fatty acid silver salt grains discharged from the closed mixing means. In such apparatus, the closed mixing means is preferably a mixing apparatus having rotating blades in a closed vessel. A linear velocity at the outermost periphery portion of such rotating blades is preferably 1 to 50 m/second, and a stirring power of such mixing apparatus is preferably 0.1 to 10 kW per liter of a reaction mixture.